1. Field of the Invention
This invention relates to an improved form of a tonometer in which an optical sensing system is used to measure pressure exerted on a probe. It also relates to an improved optical sensing system especially adapted for use in such a tonometer. More particularly, it relates to an improved hand held pressure probe for use in a tonometer of the applanation type and to a sensing system for use with the probe that detects signal levels generated by the probe in an improved manner.
2. Description of the Prior Art
Tonometers of the applanation type employing a hand held probe having a tip which is applied to the cornea of the eye or other surface of a body having an internal pressure to be measured are known in the art. For example, U.S. Pat. No. 3,992,926, issued Nov. 23, 1976 to Berryhill, discloses a tonometer of this type, which is commercially available from the assignee of this application. The probe of the Berryhill tonometer incorporates a movable rod which is displaced by intraocular or other internal pressure. The rod is connected to a capacitor structure, the capacitance of which is altered by the displacement of the rod. Measurement of the capacitance change is used to provide a display of the pressure being measured.
Due to its ease of use by personnel without specialized training, the Berryhill tonometer has achieved substantial acceptance in the marketplace. However, fabrication of the capacitor structure in this probe is somewhat difficult, due to the necessity of providing carefully fabricated capacitor plates at a predetermined spacing of about 0.001 inch. Such tonometer probes incorporating capacitive elements are also somewhat sensitive to damage by shock, such as when dropped or subjected to other impact. Relatively low yields are obtained in fabrication of these probes due to dust, moisture and capacitor plate misalignment. Dust and moisture also contribute to failure in use.
A variety of optical sensing devices are also known in the art. For example, commercially available source and detector assemblies (SDAs), incorporating infrared-emitting diodes and phototransistors, are obtainable from General Instruments Corporation under the designation MCA or Texas Instruments, Incorporated, under the designation TIL. As employed in the prior art, such SDAs are generally utilized in systems where the presence or absence of signal outputs are detected to establish an ON or OFF state, and determining the signal level is not necessary. In those applications where signal level output from the SDAs is required, two signal level detecting amplifiers have been required, with a non-inverted signal input being provided to one of the amplifiers and an inverted signal input being provided to the other amplifier. Prior art attempts to sense the signal level outputs with a single amplifier have been unsuccessful due to the presence of excessive noise in the amplifier output. The necessity of utilizing two amplifiers has prevented the use of such SDAs in situations where a limited amount of space is available for the signal level determining circuit.